This invention relates to the conversion of phthalic acids to cyclohexanedicarboxylic acids by catalytic hydrogenation. Cyclohexanedicarboxylic acids are used in the manufacture of polyesters, polyamides, resins, and coatings. The art teaches that cyclohexanedicarboxylic acids can be prepared by the direct hydrogenation of phthalic acids in aqueous solution over a supported rhodium catalyst.
According to Freifelder, et al., (J. Org. Chem. 31, 3438 (1966) phthalic acids can be hydrogenated in high yield in the presence of 5 wt. % rhodium on carbon catalyst.
U.S. Pat. No. 4,754,064 describes the use of 5 wt. % rhodium on carbon catalyst at a temperature range of 90.degree. C. to 140.degree. C. with the improvement of recycle of 5 to 25 wt. % of the product solution. The high cost of rhodium is a disadvantage of this method. The limited solubility of isophthalic and terephthalic acids at the low reaction temperatures is also a disadvantage in a commercial process.
U.S. Pat. No. 2,828,335 describes the hydrogenation of phthalic acid salts in high yield with good selectivity in the presence of a supported ruthenium catalyst. This patent also discloses the hydrogenation of phthalic acids using a variety of transition metal catalysts leading to extensive decarboxylation to give cyclohexanecarboxylic acid.
U.S. Pat. No. 5,118,841 and U.S. Pat. No. 5,202,475 also teach the hydrogenation of phthalic acid salts in the presence of supported ruthenium catalysts. A disadvantage to the hydrogenation of phthalic acid salts is that they must be treated with a mineral acid in order to recover the cydohexanedicarboxylic acid. The acidification procedure inherently generates a salt, which must be disposed of.
U.S. Pat. No. 3,444,237 shows the loss of selectivity and the formation of several byproducts when trimellitic acid is hydrogenated instead of the alkali salt.
The need exists for a process for the production of cyclohexanedicarboxylic acids, which is economically feasible and overcomes the deficiencies of the prior art.